The rapid decrease in viscosity of liquids with increasing temperature is well-known. Ideally, for many applications (automobile lubricants, etc.) it would be desirable to solve this problem so that viscosity would be insensitive to temperature. Alternatively, it might be desirable to provide liquid systems whose viscosities actually increase with temperature. It is true that with selected polymeric additives, it has been possible to reduce substantially the viscosity change with temperature which does occur with most oils and similar systems. These polymer additives, known as viscosity index improvers (or V.I. Improvers) are generally high molecular weight polymers.
The way in which these additives function can be summarized very briefly. In effect, they perform two functions, i.e., thickening, which merely increases fluid viscosity; and Viscosity Index (V.I.) improvement, which corresponds to limited thickening at ambient temperatures and a correspondingly greater thickening at elevated temperatures. This can be accomplished by utilizing a polymeric additive which is poorly solvated by the liquid at ambient temperatures; however, at elevated temperatures the polymer is more highly solvated such that the polymer expands and is a relatively more effective thickener.
While these V.I. Improvers have proven successful commercially, it is important to note that their effect at reducing viscosity changes with temperatures is rather mild. For a typical base oil containing a suitable V.I. Improver, the kinematic viscosity will still decrease by a factor of from 5 to 10 as the temperature increases from 30.degree. to 100.degree. C. Obviously, if it is desired to hold the viscosity roughly constant with such temperature changes, current technology has not offered an appropriate additive system.
U.S. Pat. No. 3,396,136 describes how copolymers of alkenyl aromatic sulfonic acids, when properly neutralized, can be employed as thickeners for nonpolar solvents. Those metal sulfonate systems have been shown to be very effective; however, when employed as two component systems (i.e., ionic polymer plus nonpolar solvent), the variation of viscosity with increased temperature is very conventional and predictable. That is, the solution viscosity decreases markedly as temperature is increased.
U.S. Pat. No. 3,396,136 further teaches "in situ" neutralization of the sulfonic acid polymer which, under some conditions, can result in the availability of a small amount of polar cosolvent--i.e., a solvent for the sulfonate groups about equal in amount to the amount of sulfonate groups which are present. This amount of polar cosolvent is not within the limits of the instant invention, which only optionally erequires amounts of the third component (which interacts with the ionomeric groups of the ionomer copolymer) at levels whch range from 10 to 600 times the molar equivalence of ionic groups. This level of cosolvent is about one to two orders of magnitude or more higher than employed in the cited art. In addition, the cited patent is restricted to aromatic sulfonate polymers. The instant invention describes other polymers such as sulfonated ethylene propylene terpolymers, sulfonated Butyl, etc., which are a portion of the polymer complex.
U.S. Pat. No. 3,366,430 teaches the gelling of organic liquids by the interaction of polar "associative bonds" which includes hydrogen bonding and "ionic cross-linking". Again, this patent specifies that two components are necessary--the associating polymer (or polymers in some cases) and the nonpolar organic liquid. There is no mention of a third polar cosolvent except to point out that such polar liquids should not be present. Specifically, this patent states (Column 2, line 7) that the hydrocarbon liquids to which this invention is to be applied should not contain a substantial portion of a miscible protolytic liquid such as methanol. It is clear that the language of this patent limits this invention to gels and further, that any amount of polar liquids which are present to an extent where they disrupt those gels are undesirable. The instant invention is distinct from that cited in that amounts of such polar compounds, as will break up gel at ambient conditions, are required and in fact the most preferred state is free of any said gel at ambient temperatures.
U.S. Pat. No. 3,679,382 teaches the thickening of aliphatic hydrocarbons with synthetic organic polymers which contain olefinically unsaturated copolymerizable acids, amides, hydroxyacrylic esters, sulfonic acids, etc. It is emphasized in this patent (Column 3, line 72) that it is critical that in the preparation of such polymers, no surface active agent, catalyst or other additive be employed which introduces a metallic ion into the system. Therefore, it is preferred to employ ammonium or amine salts. It is clear that this invention (U.S. Pat. No. 3,679,382) specifically precludes the use of metallic counterions--and is directed towards amine or ammonium derivatives. Only metallic counterions are effective in the instant invention--and that, in fact, attempts to employ amine derivatives have not resulted in the results which are the objectives of this invention. Finally, this cited patent does describe (Column 7, lines 13-19) that the addition of alcohols will reduce the viscosity of the thickened hydrocarbon and alter flow characteristics thereof.
U.S. Pat. Nos. 3,931,021 and 4,118,361 describe the use of ionic polymers and required cosolvents in an organic liquid and V. I. Improvers. The instant invention represents an improvement over U.S. Pat. Nos. 3,931,021 and 4,118,361, and therefore provides a new dimension in viscosity control of hydrocarbon-based solvents. Specifically, it has been discovered that these complexes offer a "flatter" viscosity-temperature relationship than do the sulfonate ionomers previously disclosed. Furthermore, the types of ionic polymers previously described as viscosifiers for oils and low polarity diluents usually are effective thickeners at modest levels, but if one attempts to make a concentrate (10% polymer by weight) the resulting solution is too viscous to handle. The solutions described in this invention can have relatively low viscosities at high concentrations of polymer, yet maintain relatively high viscosities at low polymer concentrations. This change in the viscosity--concentration relationship is a fundamental discovery of potentially great practical relevance.